Prussian Blue Analog as a Functional Additive for Restoring Sulfide Solid Electrolytes: Enhancing Moisture Stability in All‐Solid‐State Batteries

Abstract Sulfide‐based solid‐state electrolytes (SSEs) hold significant potential for all‐solid‐state batteries (ASSBs), but their severe degradation upon exposure to moisture remains a major barrier to their practical application. In this work, Prussian blue analogs (PBAs) are introduced as functional additives into the SSEs to enhance moisture stability. It is demonstrated that PBAs can not only suppress moisture‐induced degradation of SSEs but also actively reverse it, offering a reviving function that restores the electrochemical performance of previously degraded SSEs. When PBAs are simply postmixed with previously degraded SSEs under 5% relative humidity for 6 h, the resulting cells exhibited an average Coulombic efficiency of 99.9% over 500 cycles with 95.2% capacity retention. This restoring behavior is attributed to the open‐framework structure of PBAs, which provides abundant interstitial and coordination sites that facilitate the efficient absorption of hydrated water molecules and evolved gases such as H 2 S. In addition, PBAs exhibit a ductile nature that mitigates the increase in interfacial resistance typically induced by conventional additives, thereby preserving high ionic conductivity of SSEs. These results highlight PBAs as next‐generation multifunctional additives―capable of both protecting and recovering sulfide SSEs, offering a viable route toward stable and durable sulfide‐based ASSBs assembled under ambient conditions.